The present invention relates to a cooling unit for facilitating the radiation of heat from a heat-generating component, e.g., a semiconductor package, and also relates to an electronic apparatus incorporating the cooling unit.
Electronic apparatuses, e.g., portable computers and workstations, have a CPU each. The CPU is designed to process multimedia information, such as characters, speech, sound, and images. The data-processing speed and the number of functions of the CPU have continuously increased. The higher the speed the CPU processes data and the more functions it performs, the more electric power it consumes. The amount of heat the MPU generates while operating increases in proportion to the power it consumes.
In order to guarantee a stable operation of the CPU, a heat-radiating, cooling module such as a heat sink is indispensable. This is because the cooling module can cool the CPU with high efficiency.
Conventional heat sinks have a heat-receiving section and a heat radiating section each. The heat-receiving section receives the heat generated by a CPU. The heat-radiating section radiates the heat transmitted to the heat-receiving section. Such a heat sink is secured by screws to the housing of an electronic apparatus or to the circuit board provided in the housing and having a CPU mounted thereon. Thus, the heat-receiving section of the heat sink is thermally connected to the CPU.
A semiconductor package for use as a CPU in portable computers is a BGA (Ball Grid Array) package in most cases. The BGA package has a wiring substrate made of synthetic resin and an IC chip mounted on the wring substrate and connected thereto by flip-chip method. When the BGA package is mounted on a circuit board, its height may vary by 0.25 mm at most. Since the heat sink is an injection molding made of aluminum, i.e., a metal excelling in thermal conductivity, its parts, including the heat-receiving section, need to have dimensional tolerances.
In view of this, a heat-conducting sheet is interposed between the IC chip and the heat-receiving section of the heat sink. The sheet is made of rubber that has high thermal conductivity. The heat-conducting sheet is clamped between the IC chip and the heat-receiving section and elastically deformed when the heat sink is secured to the circuit board or housing of the electronic apparatus. Thanks to the deformation of the sheet, the change in the height of the BGA package and the change in size of the heat sink are compensated for. As a result, the heat-receiving section and the IC chip can remain in a stable thermal connection.
This conventional thermal connection between the heat sink and the BGA package is, however, disadvantageous. When the heat sink is secured to the circuit board or housing of the electronic apparatus, its heat-receiving section is pressed directly onto the heat-conducting sheet and, hence, indirectly onto the IC chip of the BGA package. In other words, the force the screws apply, fastening the heat sink to the circuit board or housing of the electronic apparatus, act as a direct stress on the BGA package. If the BGA package is strong enough to overcome this stress, no problems will arise at all. In fact, the BGA package can hardly be said to withstand the stress since the IC chip is exposed outside the package and the wiring substrate supporting the IC chip is made of soft synthetic resin.
Thus, the stress concentrates on the IC chip once after the heat-receiving section of the heat sink is thermally connected to the IC chip of the BGA package. The IC chip may therefore be broken. Moreover, a load is imposed, pushing the IC chip to the wiring substrate. This load acts a bending stress on the wiring substrate, deflecting or warping the wiring substrate. Consequently, a stress keeps acting on the junction between the IC chip and the wiring substrate. This may results in an inadequate electrical connection between the IC chip and the wiring substrate.
Hence, the load that can be applied to the heat sink to thermally connect the heat-receiving section to a semiconductor package such as a BGA package is limited. That is, the heat-receiving section of the heat sink cannot be pressed with a large force onto the semiconductor package. High thermal resistance is likely to develop at the junction between the heat sink and the semiconductor package. Heat cannot be efficiently transmitted from the semiconductor package to the heat sink.